1. Field of the Invention
This invention relates to the determination of the percentage volume of total particulates in a liquid sample utilizing an ion-selective electrode and a reference electrode.
2. Prior Art
The use of ion-selective electrodes for the measurement in whole blood or other liquid samples of the concentration of dissolved constituents such as sodium, potassium and calcium, for example, is well known. It is also known that such samples may be "spiked" with a diluent containing the same ion to which the electrode is selective in the aforementioned measuring process. This is commonly known as "standard addition analysis".
The determination of the percentage volume of total particulates in a liquid sample is useful for many purposes. For example, the percentage volume of red cells in blood, known as hematocrit, is an important clinical parameter for diagnostic use. The percentage volume of all cells in blood in most cases is essentially equivalent to hematocrit, and is also of interest though it includes the percentage volume of white cells. However, in cases of persons with relatively high white cell counts, e.g., persons having leukemia, the percentage volume of total cells and hematocrit are not essentially the same. This limitation is recognized and it is believed that it does not represent a serious problem as a white cell count determination of such blood may be made in a conventional manner.
Heretofore, hematocrit has been most commonly determined by centrifuging in a capillary tube a volume of whole blood to separate the blood into a cell portion, a heavier phase, and a serum or plasma portion, a lighter phase. This is followed by estimation, using a manual technique, of the total blood volume and the cell volume, or by automatic determination as in accordance with the teaching of Adler et al. U.S. Pat. No. 3,684,450. Colorimetric determination of hematocrit is described by E. Ponder: Hemolysis and Related Phenomena, pp 51-53, Grune and Straton, N.Y., 1948. Such colorimetric determination also requires, among other things, such as dye addition, centrifuging of a portion of the sample. Determination of the percentage volume of blood cells by conductimetric techniques is also known. Beaver U.S. Pat. No. 3,648,260 describes two such techniques, the preferred and most accurate one of which also requires centrifuging of a sample portion. The other technique described by the last-mentioned patent does not require centrifuging of the sample, but this technique requires an assumption of a mean value for the conductivity of plasma. The conductivity of plasma arises from the presence of ions produced by dissociation of salts, e.g., sodium and chloride ions. Variation in the conductance of plasma, as found in at least certain diseased states, introduces significant errors in such a cell volume determination. E. Ponder, Supra, also describes, among the general types of techniques discussed above, diffractometric and photographic determinations (pp 62-79) of hematocrit, which are both optical techniques not requiring centrifuging of a whole blood sample, utilizing a diffraction pattern and a photographic image, respectively, of a thin film of cells to estimate the mean cell diameter and volume. Both the diffractometric and photographic methods require a red cell count of the blood sample as by inspection of a thin film of blood under a microscope for example. Hematocrit is then calculated from the estimated values of mean red cell volume and red cell count. Yet another method of determining hematocrit involves passing the diluted blood sample through a small orifice and measuring the electrical resistance changes which occur across the orifice. The frequency and magnitude of such changes are related to the cell count and to the mean cell volume, respectively, from which parameters hematocrit is calculated.
The present invention seeks to overcome difficulties encountered with such prior techniques of determining the percentage volume of total cells in blood, in large part by avoidance of centrifuging or determining the red cell count of a sample of whole blood which is time consuming and requires relatively expensive apparatus.